1. Field of the Invention
The invention relates to the field of micromachined virbratory gyroscopes, in particular gyroscopes with a one degree-of-freedom drive mode and a two degree-of-freedom sense mode.
2. Description of the Prior Art
The operation of all micromachined vibratory gyroscopes is based on a transfer of energy between two modes of vibration caused by the Coriolis effect. Conventional implementations often utilize single degree of freedom drive and sense-modes. Several of these conventional implementations reported gyroscopes with structurally symmetrical designs aimed at mode-matched operation. In such implementations, the mechanical gain is increased proportionally to the sense-mode quality factor. Additionally, mode-matching feedback control can be employed to improve sensitivity by electronically tuning the drive- and sense-modes. Alternatively, parametric excitation of the drive-mode providing large amplitudes over a wide range of frequencies can be used to eliminate the frequency mismatch between the drive- and sense-modes. However, mode-matched operation has practical challenges, requiring precise matching of the operational modes over wide temperature ranges. As a result of mode-matched operation, the increased sensitivity is achieved at the cost of sensor robustness, temperature bias drift, bandwidth and linear operational range.
Alternatively, the modes of operation can be designed with a certain frequency mismatch. Even though this approach improves the robustness and the bandwidth characteristics, the improvements are limited due to the dimensionality of the design space. Restrictions of design approaches with single-DOF drive- and sense-modes dictate a tradeoff between achieved robustness/bandwidth and gain.
Structural design approaches leading to robust gyroscopes are an intriguing option to consider. Several approaches have been previously explored including a design of a non-resonant gyroscope with 2-DOF drive- and 2-DOF sense-modes, and a gyroscope design with a 2-DOF drive-mode and 1-DOF sense-mode. Previously reported designs illustrated that the increase of system dimensionality and careful selection of system parameters may lead to an increase of system robustness. However, increasing the number of degrees of freedom in the drive-mode may not be the best choice as it requires actuation of the drive-mode at a non-resonant frequency, which is less efficient than resonant actuation.
For increasing robustness of vibratory gyroscopes, it is beneficial to design 1-DOF drive- and 2-DOF sense-modes so that the drive-mode resonant frequency is placed between the two resonant peaks of the sense-mode. A single-DOF drive-mode is more suitable for resonance-locking closed loop operation, similar to conventional gyroscopes. A 2-DOF sense-mode provides extended design flexibility and robustness by utilizing a dynamically coupled response when the drive-mode is operated in between the two coupled resonant peaks. For example, one such design found in the prior art has been demonstrated to provide robust operation with a 200 Hz bandwidth using a micromachined prototype with a 750 Hz operational frequency. For this design concept, increasing the operational frequency would further increase the bandwidth, while also resulting in a decrease of the sensitivity.
Most real-world applications such as automotive, military, and consumer electronics require robust yet sensitive gyroscopes with operational frequencies above several kHz in order to suppress the effect of environmental vibrational noise. At the same time, the desired mechanical bandwidth of the sense-mode is typically above 100 Hz, but not more than 400 Hz.
Previous gyroscopes employing a 2-DOF sense-mode rely on a dynamic vibration absorber (DVA) structure, in which the frequency response characteristics strongly depend on both the operational frequency and the ratio between the smaller and the bigger sense-mode masses. In this case, the gain of the gyroscope is inversely proportional to the spacing between the sense-mode peaks. Adapting the DVA-based gyroscope design for operational frequencies above 1 kHz while maintaining the sense-mode peaks at a practical spacing is challenging due to the limitation of the design space and involves a stringent tradeoff between the die size and detection capacitance.
Therefore, a new gyroscope design concept is desired which would preserve the advantages of the multi-DOF concept, while eliminating the scaling tradeoff and allowing flexible selection of required bandwidth and arbitrary high operational frequencies.